1. Field of the Invention
This invention relates to polyphase variable speed "brushless direct current (DC)" motors and, in particular, to speed controls for such motors.
2. Description of the Related Art
Brushless DC machines are well known in the art. A brushless DC machine is a sychronous machine, which is powered by alternating current, operated in such a way as to behave like a DC machine. Input power from a DC source is converted into alternating currents which are supplied to armature windings on a stator. Sensors signal the position of the rotor, on which magnets are located, to electronics which control switching elements converting the input DC power to polyphase alternating currents. The frequency and phase angle of the stator currents are controlled such that a constant angular displacement exists between the poles of the rotating stator field and the field poles of the rotor. Such a constant angular displacement also exists in a DC machine. However, in a DC machine the field windings are on the stator and the armature windings are on the rotor.
Typically, the polyphase alternating currents are three-phase currents, the phases being displaced from one another by a phase angle of 120.degree.. Each phase of the three phases of alternating current supplies one of three phase windings found on the stator. The alternating current supplied to the phase windings can be either bipolar or unipolar. Bipolar alternating current can be a full rectangular wave or sinusoidal alternating current and unipolar alternating current is usually a half rectangular wave alternating current, the current varying from zero to a certain magnitude with no current reversal. A typical three phase winding for a three phase bipolar brushless DC motor in a star configuration is shown in FIG. 1. Typical examples of bipolar and unipolar alternating current are shown in FIGS. 1a and 1b, respectively.
In the configuration shown in FIG. 1, two phase windings can be connected to the power supply through the half H-bridge switching system comprised of at least one half H-bridge switching arrangement 2. For example, to turn phase windings A and B on, switches Q1 and Q4 would be turned on. To apply current in the opposite direction through phase windings A and B, switches Q3 and Q2 would be turned on. The remaining switches would be left in the open position. V.sub.S stands for supply voltage and GND stands for ground in FIG. 1.
The current flow in each winding must be synchronized to change direction simultaneously with the change in direction of the back electromotive force (EMF) developed by the phase winding such that the current flow in each winding opposes the back EMF developed by each phase winding. FIG. 2 illustrates the variation over time of the back EMF for each phase winding and the combined variation of back EMF and current over time for any two of the three phase windings shown in FIG. 1.
Current always flows through two phase windings when the combined phase to phase back EMF is at its maximum magnitude, shown in FIG. 2 by the flat portion of a trapezoid. The sequence in which all three phases are energized is shown in FIG. 2a. FIG. 2a presents one electrical cycle that is continuously repeated as the motor rotates. When the back EMF is reversed in a pair of coils, the current must reverse its direction as well. This is effected by turning on corresponding switches.
When operating in the above mode, the motor will run at full speed, determined by the applied load. One of the important features of brushless DC motors is the ability to change the operating speed continuously. Speed control has been achieved by various methods including linear speed control, pulse width modulation (PWM), block commutation, and by applying resonant converters.
Speed variation using linear speed control is obtained by operating switches, for example, metal-oxide semiconductor field effect transistors (MOSFETs) in their linear operation regions (as variable resistors). The MOSFETs' resistance determines the magnitude of current passed through the phase coil. This type of control can only be utilized for relatively low levels of current due to the MOSFET heating effect.
Speed variation using PWM is obtained by chopping the phase current. The PWM frequency is much higher then the EMF waveform frequency. In this case, the current pulses of a constant frequency are varied in width, thus changing the average value of the current. MOSFETs can act as switches which are either fully turned on or fully turned off. When the MOSFETs are turned off, the phase current is chopped. The pulse width of 100% corresponds to full speed operation. The lower the pulse width, the lower the average phase current and speed. A pulse width of 60% means that the MOSFET is turned on for 60% of the pulse duration time, and is turned off for the remaining 40% of the time. This type of control, although more efficient than linear speed control, requires a snubber circuit comprising a power resistor (dissipating substantial amounts of heat) and a capacitor. It can also generate substantial amounts of radio frequency interference due to fast current transition rates.
Speed variation using block commutation is obtained by changing the duration of the phase current during each conduction period, see FIG. 3. In this type of control, speed is a function of time during which the relevant phase MOSFETs are turned on for each conduction period, a conduction period being, for example, 60 degrees. In such a case, the phase duration would be in the range of approximately 20 degrees for low speed, 40 degrees for medium speed, and 60 degrees for full speed operation. The main advantage of this method is a simple drive circuit, with relatively high efficiency. The disadvantage is the fact that at low speeds, the magnitude of current pulses can become high. As a result, this method has been used only over small ranges of speed.
Speed control using resonant converters is obtained using various control topologies in which resonant components such as inductors and capacitors are used to control the energy flow. This is an emerging method of speed control by which very low thermal losses and reduced radio frequency interference (RFI) can be obtained. Added components may limit this technology in space and cost sensitive applications.